Apparatus for controlling and regulating a cutter hoist

ABSTRACT

Motor control system for a mining machine of the type in which a cutting element is driven by a first electrical motor and the mining machine is advanced into a coal face by a second motor. The system is characterized in that the torque and speed of the second motor are reduced both when the cutter element drive motor is idling as well as when the cutter motor is overloaded. The arrangement is such that as the cutting element approaches the coal seam and is idling, the speed of the advancing drive motor is relatively low; when the cutter enters the coal face and the current to the cutter drive motor increases, the speed of the advancing drive motor is also increased; and when the cutter element drive motor is overloaded, the advancing drive motor is reversed to withdraw the cutting element from the coal face and thereafter again advances the cutting element into the coal face in a pulsing or reciprocating motion.

United States Patent Inventors App]. No.

Filed Patented Assignee Priority APPARATUS FOR CONTROLLING AND REGULATING A CUTTER HOIST l 1 Claims, 2 Drawing Figs.

U.S. Cl 318/39, 173/7, 299/1 Int. Cl B23q 5/28 Field of Search 318/39, 68;

THRESHOL D 05 TEL ran 22 VOL TA 65 REGULA TO)? VOLTAGE FEHENCE SEE l/O MU TOR CON TROL R-C TWORK TIME DELAY CURRENT L/H/TER [56 References Cited UNITED STATES PATENTS 2,129,049 9/l938 Doran 318/39 x 2,809,333 lO/l957 Wagner 318/39 Primary Examiner-Benjamin Dobeck A Assistant ExaminerW.E. Duncanson, Jr.

Att0meyBrown, Murray, Flick & Peckham ABSTRACT: Motor control system for a mining machine of the type in which a cutting element is driven by a first electrical motor and the mining machine is advanced into a coal face by a second motor. The system is characterized in that the torque and speed of the second motor are reduced both when the cutter element drive motor is idling as well as when the cutter motor is overloaded. The arrangement is such that as the cutting element approaches the coal seam and is idling, the speed of the advancing drive motor is relatively low; when the cutter enters the coal face and the current to the cutter drive motor increases, the speed of the advancing drive motor is also increased; and when the cutter element drive motor is overloaded, the advancing drive motor is reversed to withdraw the cutting element from the coal face and thereafter again advances the cutting element into the coal face in a pulsing or reciprocating motion.

TNYR/S TOR F/H/IVG CIRCUIT CURRf/VT LIN/TE)? HOLDING 25 TH YRISTOR Fl RING CIRCU/ 7' APPARATUS FOR CONTROLLING AND REGULATING A CUTTER HOIST BACKGROUND OF THE INVENTION While not limited thereto, the present invention is particularly adapted for use in controlling the drive motors for a longwall mining machine. In the mining of coal veins by the longwall mining method, a cutting machine is employed that cuts along the longwall face of the coal seam while moving longitudinally along the longwall face on a guideway or track placed therealong. A suitable cutting machine for performing this operation usually consists of a machine body movable along the guideway, and one or more support arms pivotally attached to the body and carrying rotary cutting tools.

A longwall mining machine of this type is usually advanced by means of a winch arrangement comprising a chain wheel which is in engagement with a chain looped about a guide chain wheel and anchored at its opposite ends to the ends of the longwall face. With this arrangement, rotation of the chain wheel by a drive motor will cause the mining machine to advance along the tracks on which it is mounted. This motor can be referred to as an advancing drive motor; while the motor or motors which drive the cutting element can be referred to as a cutter drive motor. It will be appreciated that with this arrangement, the load on the cutter drive motor will be a function not only of the hardness of the coal being mined, but also the speed of advance of the cutter into the coal face as determined by the speed and torque of the advancing drive motor. Thus, the operation of the two motors must be correlated in order to prevent overloading of the cutter drive motor. Furthermore, when the cutting element is not engaging a coal face, it should be running at idling speed. As the cutting element approached and engaged the coal face at the idling speed its speed and torque should be increased as well as the speed of advance of the mining machine. This, of course, likewise means that the operation of the advancing drive motor must be correlated with the cutter drive motor in order that when the cutting element engages a coal face, the speed of the advancing motor will be increased. At the same time, if an overload condition should occur on the cutter drive motor, the advancing drive motor should be reversed to withdraw the cutting element from the coal seam, followed by advancement of the cutting element back into the coal seam.

SUMMARY OF THE INVENTION In accordance with the present invention, a motor control system for a mining machine is provided wherein the operation of a cutter drive motor and a mining-machine-advancing motor are correlated such that sudden load increases on the cutter element motor and advancing drive motor, which can occur with an abrupt entering of the cutting element into the working face, are avoided to a great extent. Moreover, the load variations which occur during normal cutting operations and which are caused by the variable hardness of the coal being mined are constantly restored to a manually set nominal value.

The foregoing is accomplished in accordance with the invention by a motor control servo loop for a direct current advancing drive motor including the usual voltage regulator and current limiter, the output of which drives thyristor firing circuits for a dual-bridge thyristor power supply which supplies driving potential to the advancing drive motor. The current through both the cutter drive motor as well as the advancing drive motor is sensed; and should the current through either motor exceed a predetermined value, indicating an excessive torque on the motor, the current limiter reduces the speed of the advancing motor and, hence, the load on both motors.

The control for the advancing motor is such that a nominal predetermined advancing speed is established when no load is on the cutter drive motor. However, when the cutting element or elements enter a coal seam and the load on the cutter drive motor increases, the increased current through the motor is sensed by a threshold detector which causes the motor control system to increase the speed of the advancing motor. Thus, the cutter element approaches the coal seam gradually; and as it engages the coal face the load thereon increases, thereby increasing the advancing speed of the mining machine while avoiding abrupt, excessive torques on the cutter drive motor which would occur if the cutting element entered the coal seam at higher speeds.

During a mining operation, the cutting element may incur certain obstacles such as props, cross timbers and the like, which act to overload either or both the cutter drive motor and the advancing motor. Such an overloading occurs suddenly and persists for a given time period as contrasted, for example, with a momentary overload which may occur due to various factors. Accordingly, a second current-limiting circuit in the motor control system which, in response to an excessive overload condition for a predetermined period of time, will cause the current to the armature of the direct current advancing motor to become reversed, whereby the cutting element will be withdrawn from the coal face. Upon the withdrawal from the coal face, the speed of the advancing motor is reversed, and the cutter element again caused to approach the coal face. If the obstacle no longer exists, the operation of the machine proceeds as usual. However, if the obstacle again creates an excessive overcurrent condition, this second overcurrent condition is sensed and causes the control circuitry for the advancing motor to stop the same. In this manner, a continuation of the forward and reverse motion of the mining machine in response to an obstacle which cannot be readily overcome is prevented.

The system of the invention additionally includes over temperature protection systems effective on the current limiter of the main control loop for the advancing motor to reduce its speed. Finally, in order to prevent an excessive speed of the mining machine into a soft coal seam, the speed of the advancing motor is monitored; and when it exceeds a maximum permissible value, the firing angle of thyristors supplying power to the field winding of the drive motor is reduced until the speed of the motor is again brought back to a permissible value.

DESCRIPTION OF PREFERRED EMBODIMENT The above and other objects and features of the invention will become apparent from the following detailed description taken in connection with the accompanying drawings which fonn a part of this specification and in which:

FIG. 1 is a schematic illustration of one type of mining machine with which the present invention may be used; and

FIG. 2 comprises a schematic circuit diagram of the motor control system of the invention.

With reference now to the drawings, and particularly to FIG. 1, the arrow A denotes the direction in which a mining machine B, movable on tracks or guideways C, advances along the face D of a mineral seam which, in the usual case, is a coal seam. The mining machine B is provided with a rotary cutting element E that cuts and removes the coal and exposes a new coal face F when the mining machine B advances along the coal face D that has been exposed by a preceding cut. The mining machine is driven by means of an advancing direct current drive motor 3 connected to a winch-type chain wheel H around which a chain J passes. The chain J is secured at one end of the longwall coal face D as at K; and its other end is secured at the opposite end of the longwall coal face as at L. The chain J extends from point K, around the driving chain wheel H, thence around a guide wheel M to point L where it is secured. With the arrangement shown, it can be seen that as the advancing motor 3 rotates in one direction, the mining machine B will be caused to move in the direction of arrow A; whereas when the drive motor 3 is reversed, so also is the direction of movement of the mining machine. The rotary cutting element E is driven by means of a three-phase alternating-current motor 2. The cutting element E is usually carried on a pivoted arm such that it can be raised or lowered; and in many cases the mining machine will incorporate two cutter elements on pivoted arms, one at the front and one at the back, such that the forward cutting element can work the floor of the seam while the rear element works the upper seam portion. In that case, both cutter elements are driven by the single motor 2.

In the operation of a mining machine such as that shown in FIG. 1, the motor 3 will cause the mining machine to advance to the right along the direction of arrow A; while the cutting element E rotates and removes an entire layer of the longwall face. As was mentioned above, the cutting element E is pivotally mounted such that it can be raised or lowered; and, thus, two or more excursions of the mining machine can be made to remove the entire height of a coal face, depending upon its height. After the entire face is removed, the tracks C are then moved upwardly as viewed in FIG. 1, whereupon the process is repeated to remove a succeeding layer of coal.

It will be apparent that wi h a mining machine of the type shown in FIG. 1, the current through the motors 2 and 3, and hence the torques thereon, will be dependent not only upon the rate of movement of the mining machine B but also the hardness of the material being mined. As the speed of the mining machine is increased, the current through the motor 2 will increase; and should the current become excessive, it can be corrected by reducing the speed of motor 3 and, hence, the rate of advance of the mining machine B. Furthermore, in order to avoid sudden current surges in the cutter drive motor 2, the cutting element E should rotate at a relatively low speed when advancing into the coal face. At this time, the mining machine should also advance the cutting element into the coal face at a relatively slow speed, followed by an increase in the speed of advance of the machine. All of this must be accomplished while avoiding excessive current surges through the motors 2 and 3 due to varying conditions of the coal being mined.

The control circuitry for the motors 2 and 3 is shown in FIG. 2 and includes the three conductors or phases R, S, and T of a three-phase power supply, not shown. The conductors R, S and T are connected through master switch 1 to the threephase altemating-current motor 2 which is designed to rotate at a constant rate of speed. This speed can be varied by external control circuitry, not shown, however during a mining operation it will ordinarily rotate at one selected speed.

The direct current advancing motor 3 is powered by means of a pair of balanced thyristor bridges 19 and 19 adapted to supply current through the armature of motor 3 in either direction and, hence, cause its direction of rotation to reverse. The junctions of the diodes in the two thyristor bridges 19 and 19 are connected to the same three-phase power conductors R, S and T as is the altemating-current cutter drive motor 2.

The motor control servo loop for the motor 3 includes the usual voltage regulator 14 connected at its output to a current limiter 15. The current limiter 15, in turn, is connected through switch 17 to two thyristor firing circuits l8 and 18 for the dual bridge thyristor power supplies l9 and 19', respectively. Connected to the shaft of motor 3 is a tachometer generator 12 connected through lead 13 back to a summation point 11 where it is compared with the output of a voltage reference controller 10. The voltage reference controller 10, in turn, generates a reference voltage which is compared at summation point 11 with the feedback voltage from tachometer generator 12 to produce an error signal for the voltage regulator 14.

As long as the feedback signal from the tachometer generator 12 is equal to the output of the voltage reference controller 10, the two signals will cancel at summation point 11 and the speed of the motor 3 will remain constant. However, if the feedback signal from tachometer generator 12 is below the output of the voltage reference controller 10, then an error signal will be developed to cause the voltage regulator 14, through current limiter 15, to increase the tiring angle of the thyristors in thyristor units 19 and 19 to increase the speed of the motor. Similarly, when the signal or lead 13 from tachometer generator 12 is above that from the voltage reference controller, meaning that the speed of the motor is above that established by the reference controller 10, then an error signal of the opposite polarity will be applied to voltage regulator 14 to cause the motor speed to decrease.

One of the leads connected to the altemating-current cutter drive motor 2 is provided with a current transformer 5 con nected through rectifier 5' and summation point 31 to lead 7 which, in turn, is connected to an OR-circuit 9. Similarly, one of the leads connected to the two thyristor units 19 and 19' is provided with a current transformer 6 connected through rectifier 6' and lead 8 to the OR-circuit 9. A third input to the OR-circuit 9 is connected to the output of voltage reference controller 10. Thus, should the current through motor 2 or motor 3 exceed a predetermined maximum value, a signal will be produced which will pass through OR-circuit 9 to the current limiter 15, thereby reducing the firing angles of the thyristors in units 19 and 19' to reduce the speed of the motor 3 and alleviate the overcurrent condition.

The voltage reference controller 10 is provided with a switch, not shown, which initially establishes the rotating speed of the motor 3 when the motor 2 is idling and the cutter elementE is not loaded. Under these circumstances, the mining machine will advance to the right as shown in FIG. 1, for example, until the cutting element E engages the coal face; at which time the current through the motor 2 increases abruptly. This increase in current, in turn, is sensed by a threshold detector 22 which actuates a servomotor control 24 to cause a servomotor 21 to move the moveable wiper on a potentiometer 20 to the left as viewed in FIG. 2, thereby increasing the output of the voltage reference controller 10 and causing a corresponding increase in the speed of advancing motor 3. Thus, once the cutting element enters the coal face and the load on drive motor 2 increases with a corresponding increase in current through the motor, this is sensed by the threshold detector 22 to cause the servomotor 21 to actuate rheostat 20, whereupon the output from the voltage reference controller increases as does the speed of the motor.

The thyristor units 19 and 19' are provided with temperature sensing devices 25 which may, for example, comprise solid-state thyristors or other similar devices capable of producing an output signal proportional to temperature. A similar temperature sensing device 25 is provided for the advancing drive motor 3. The temperature sensing elements 25 are connected through leads 26, 27 and 28, respectively, to an OR-circuit 29, the output of the OR-circuit 29 being connected to the current limiter 15. In this manner, should the temperature of the thyristors in units 19 and 19', or the temperature of motor 3 exceed a predetermined value, the current limiter 15 will be actuated to again reduce the speed of the motor and prevent damage thereto because of overheating.

The cutter drive motor 2 is also provided with a temperature sensing element 25 which is connected, via lead 30, to the summation point 31. When the temperature of the motor 2 becomes excessive, a signal produced by the temperaturesensing device 25 is superimposed on the current signal derived from current transformer 5 on lead 7, thereby acting, through OR-circuit 9 to cause the current limiter 15 to reduce the speed of the motor 3 and, hence, reduce the load on motor 2 until the excessive temperature condition is eliminated.

With a controlled mining operation, the load current of the advancing motor 3 will normally not reach the nominal value which is preset manually. This is due to the fact that this nominal value is set very high, when the cutter drive motor 2 is switched ON, in order to utilize the capacity of the mining machine fully.

If the cutting element E of the mining machine enters an opening in the working face, the speed of the advancing motor 3 will increase immediately to the preset theoretical value. At the end of the opening in the face, the cutter again enters the coal seam at full speed. As a result, the motor 2 is loaded suddenly; and the current sensed by current transformer 5 increases abruptly. Such an increase in current, which may be equal to twice the normal rated current of the motor, will not have ill effects unless it persists for a period of time.

When an extreme overload condition occurs for a period beyond, for example, two seconds, a current limiter circuit 32 actuates the switch 17 to interrupt the thyristor firing circuit 18, firing the thyristors in group 19 for example, and causes thyristor firing circuit 18' to fire the thyristors in group 19' whereby the direction of current flow through the motor 3 is reversed, as is its direction of rotation. The switchover from one thyristor firing circuit to the other occurs in a period of about 20 milliseconds; however it is important that the switchover occur when the applied waveform crosses the zero axis. Accordingly, the voltage across the motor armature, appearing across resistors 34, is applied via lead 35 to the switch 17 such that the switchover from one thyristor firing circuit to the other occurs exactly at the zero crossing of the applied waveform.

When the current through the motor 3 is reversed, it operates as a generator until stopped and thereafter rotates in the opposite direction of rotation. Consequently, the mining machine changes its direction of travel, draws the cutting element back from the working face and unloads it.

After the period determined by the holding circuit 33, the motor 3 will again reverse and the cutter element will advance toward the coal face. However, when the cutter element was initially withdrawn from the coal face, the output of voltage reference controller dictated a relatively high rate of speed of the motor 3; whereas it is desired to reenter the coal face slowly. In order that the motor 3 does not subsequently move the cutting element in the opposite direction at a high rate of speed, a signal from switch 17 on lead 36, which occurs when the switch 17 is actuated by holding circuit 33, is applied via lead 37 and through R-C network 38 to the servomotor control 24 which causes the servomotor 21 to rotate potentiometer such that the output of the voltage reference controller again dictates a relatively slow advancing speed of the motor 3. For this reason, the cutting element will move back into the coal face at a slow speed, which is usual.

After the holding period determined by holding circuit 33, the motor 3 again advances the cutting element back into the coal seam under the control of current limiter 15. At this time (i.e., when the cutting element is moving back into the coal seam), the voltage detected by the threshold detector 22 is below the value which actuates servomotor 21 and, hence, the motor 3 advances the mining machine at low speed. However, the feed speed is again increased as described above when the cutting element enters the coal face and the current through motor 2 increases. This is detected by the threshold detector 22.

There may be conditions wherein the overload on the cutting element is caused by its engagement with a prop or roof timber which projects into its path of travel. In this case, the load current of the motor 2 does not increase, but that of the advancing motor 3 does to actuate the current limiter 32. Consequently, the motor 3 is reversed and the cutting element is with drawn from the coal face, whereupon it again moves against the prop or the roof timber at a low feed speed as explained above. When the cutting element again enters the coal face under these conditions, the motor 3 will again be overloaded, causing it to again withdraw from the coal face. In order to prevent a continual back and forth or oscillating motion of the mining machine under these circumstances, the signal on lead 36 which is applied to the servomotor control 24 through R-C network 38 is also applied to an AND-circuit 40 through lead 41. At the same time, it is applied through time delay circuit 39 to the other input of the AND-circuit 40. time delay of circuit 39 is such that when two back and forth motions of the motor 3 occur under the conditions described, simultaneous signals will appear at the input to AND-circuit 40, causing an output signal to appear on lead 42. This signal on lead 42 actuates the voltage reference controller 10 to stop the motor 3 through an R-C network 49. Thus, under these conditions, the advance of the mining machine is stopped until the roof timber or other obstacle is removed or other corrective action is taken.

The output of the voltage regulator 14 is also applied to a RPM monitor 43. Also applied to the RPM monitor 43 via lead 44 is the output of the tachometer generator 12 proportional to the actual speed of the motor 3. When the voltage of the tachometer generator 12, which is proportional to the speed of the motor 3, exceeds a value which is in excess of that which can be attained by means of armature control through the thyristor firing circuits I8 and 18, the RPM monitor 43 will actuate a thyristor firing circuit 46 through lead 45 to decrease the firing angle of thyristor 47, which supplies current to the field winding 48 for motor 3. Thus, the filed for motor 3 is reduced. This situation occurs if the cutter element enters a seam section with especially soft, sloping coal, which requires only a slight drive power of the cutting element and the advancing motor 3. When the excessive speed condition no longer exists, this is again detected by the RPM monitor 43, whereupon the thyristor firing circuit 46 tires thyristor 47 at the value for which it was designed, and the speed control is then achieved again by means of comparison of the feedback signal on lead 13 with the voltage reference signal from circuit 10.

Although the invention has been shown in connection with a certain specific embodiment, it will be readily apparent to those skilled in the art that various changes in form and arrangement of parts may be made to suit requirements without departing from the spirit and scope of the invention. In this respect, it will be apparent that while the invention has been shown herein in combination with a long wall mining machine, it can be used equally well with other mining machines where the loads on advancing and cutting element drive motors must be correlated.

We claim as our invention:

1. In apparatus for controlling a mining machine of the type in which the mining machine is advanced into a cool seam by an advancing electrical drive motor and a coal-cutting element on the mining machine is driven by a cutter drive motor; the combination of motor control means for said advancing drive motor including a current-limiting circuit responsive to current flow through said cutter drive motor for controlling the speed of said advancing motor such that the speed of the advancing motor will be relatively low when the cutting element is removed from a coal face and is essentially unloaded, while the speed of the advancing motor will be increased when the cutting element enters the coal face and is loaded.

2. The combination of claim 1 wherein said control means includes a voltage reference controller for producing an electrical signal proportional to the desired speed of said cutter drive motor, means for comparing said signal proportional to desired motor speed with a signal proportional to actual motor speed to derive an error signal, a voltage regulator responsive to said error signal, said current-limiting circuit being connected to the output of said voltage regulator, and a controllable power supply for said advancing drive motor connected to the output of said current-limiting circuit.

3. The combination of claim 2 wherein said advancing drive motor is a direct current motor and said cutter drive motor is an altemating-current motor.

4. The combination of claim 3 wherein said cutter drive motor is connected to a three-phase altemating-current supply which also supplies power to said controllable power supply for said advancing drive motor.

5. The combination of claim 2 including means responsive to a current surge in said cutting element drive motor or said advancing drive motor for reversing the direction of rotation of said cutter drive motor to withdraw the cutting element from the coal face.

6. The combination of claim 5 including means for disabling said advancing drive motor after said advancing drive motor is reversed in direction at least two times.

7. The combination of claim 5 wherein said means responsive to a current surge includes a second current limiter in shunt with said first-mentioned current limiter and is actuable to reverse said cutter drive motor only after said current surge persists beyond a predetermined period of time.

8. The combination of claim 2 including means in said voltage reference controller for establishing an idling speed for said cutter drive motor, a threshold detector for sensing a rise in current through said cutter drive motor when said cutter element enters a coal seam, and means coupled to the output of said threshold detector for increasing the electrical signal output of said voltage reference controller and the speed of said advancing drive motor.

9. The combination of claim 2 wherein said current limiting circuit is connected through an OR-circuit to current transformers coupled to power input leads to both said advancing drive motor and said cutter drive motor whereby a current surge is either motor will reduce the speed of said advancing drive motor.

10. The combination of claim 2 wherein said controllable power supply is of the reversible thyristor type.

11. The combination of claim 2 wherein said advancing drive motor is a direct current motor having a field winding supplied with power through a thyristor device, and means for reducing the power supplied to said field winding through said thyristor device where the speed of said advancing drive motor exceeds a predetermined limit. 

1. In apparatus for controlling a mining machine of the type in which the mining machine is advanced into a coal seam by an advancing electrical drive motor and a coal-cutting element on the mining machine is driven by a cutter drive motor; the combination of motor control means for said advancing drive motor including a current-limiting circuit responsive to current flow through said cutter drive motor for controlling the speed of said advancing motor such that the speed of the advancing motor will be relatively low when the cutting element is removed from a coal face and is essentially unloaded, while the speed of the advancing motor will be increased when the cutting element enters the coal face and is loaded.
 2. The combination of claim 1 wherein said motor control means includes a voltage reference controller for producing an electrical signal proportional to the desired speed of said cutter drive motor, means for comparing said signal proportional to desired motor speed with a signal proportional to actual motor speed to derive an error signal, a voltage regulator responsive to said error signal, said current-limiting circuit being connected to the output of said voltage regulator, and a controllable power supply for said advancing drive motor connected to the output of said current-limiting circuit.
 3. The combination of claim 2 wherein said advancing drive motor is a direct current motor and said cutter drive motor is an alternating-current motor.
 4. The combination of claim 3 wherein said cutter drive motor is connected to a three-phase alternating-current supply which also supplies power to said controllable power supply for said advancing drive motor.
 5. The combination of claim 2 including means responsive to a current surge in said cutting element drive motor or said advancing drive motor for reversing the direction of rotation of said cutter drive motor to withdraw the cutting element from the coal face.
 6. The combination of claim 5 including means for disabling said advancing drive motor after said advancing drive motor is reversed in direction at least two times.
 7. The combination of claim 5 wherein said means responsive to a current surge includes a second current limiter in shunt with said first-mentioned current limiter and is actuable to reverse said cutter drive motor only after said current surge persists beyond a predetermined period of time.
 8. The combination of claim 2 including means in said voltage reference controller for establishing an idling speed for said cutter drive motor, a threshold detector for sensing a rise in current through said cutter drive motor when said cutting element enters a coal seam, and means coupled to the output of said threshold detector for increasing the electrical signal output of said voltage reference controller and the speed of said advancing drive motor.
 9. The combination of claim 2 wherein said current limiting circuit is connected through an OR-circuit to current transformers coupled to power input leads to both said advancing drive motor and said cutter drive motor whereby a current surge in either motor will reduce the speed of said advancing drive motor.
 10. The combination of claim 2 wherein said controllable power supply Is of the reversible thyristor type.
 11. The combination of claim 2 wherein said advancing drive motor is a direct current motor having a field winding supplied with power through a thyristor device, and means for reducing the power supplied to said field winding through said thyristor device where the speed of said advancing drive motor exceeds a predetermined limit. 